The aim is to provide a better understanding of the electronic basis for the interactions of barbiturates with other molecules representative of possible drug receptors. Experimental charge density distributions in crystal structures will be determined from X-ray and neutron diffraction data by a least squares fit to a model consisting of rigid pseudoatoms. Molecules are then described in terms of a limited number of parameters (about 25 for each first-row atom), which define the nuclear framework, its thermal motion, and the aspherical charge distribution about each nuclear center. The validity of the model will be checked by calculating molecular properties dependent on the charge density. In principle, the static charge density can be deconvoluted from the crystal motion. This will allow us to make direct comparisons involving both experimental and theoretical charge densities. With structural parameters determined experimentally, it may eventually be possible to calculate the properties of noncrystalline molecular aggregations, such as barbiturate receptor models.